Super Regulator, collecting the facts

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jackinnj

jackinnj

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Joined 2002
tech-diy.com
WaltJ said:
I suggest a very simple check that could be helpful. With the spurious signals displayed, switch off the power to the reg, and see what changes.

All instances of oscillation I've seen with these regs have always been with tens if not hundreds of mV of sine wave output. I do agree with the observation that a time domain plot won't show this lower level stuff.

wj
Click to expand...

Yes, I did exactly that, but didn't take a pic -- you can still see the signal at 770kHz from WABC-AM, and a little less for WINS-AM (1,100kHz) and WFAN (660kHz). I'm not going to spend too much time on this, will measure once they are all boxed up, but I don't think it's going to be an issue. I think that with the layout on the Old Colony boards there's not enough opportunity for energy from these signals to promote oscillation.
 
jackinnj

jackinnj

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Joined 2002
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These tests were done with the Old Colony (AudioXpress) boards -- and the leads to the spectrum analyzer were my good pomona's. The radio stations showed up when the regulator was un-housed.

I would say it's a good recommendation to refrain from using a plastic or wood case for anything using an AD797 in a high gain configuration.

and just to reiterate -- i've had no issues with oscillation with the AD797 and the OCS boards.
 
jackinnj

jackinnj

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Joined 2002
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peranders said:
Which board did you use? Output voltage?
Click to expand...

I used the board which Jan designed, I will call it "the Reference Board". The output voltage was +/- 13.7 VDC and the load 137mADC + 50mA AC. Right now I have an AD825 on my Sjostrom board and I will have to get the rework station out to look at the other opamps.

You can get the Zout down into the low 20's of micro-Ohms @100Hz -- with the AD797, however, the Zo is in the low teens. 9Jung was able to measure in single-digits.)

With regard to the caps -- the Panasonic HFQ's are no longer available the 120uF/25 V had ESR of 200milliOhms, the FC's have ESR of 250milliOhms. ESR is important in the stability of the SR.
 
jan.didden

jan.didden

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ticknpop said:
Actually the film version was designed to get rid of the dielectric absorbtion of the electrolytic capacitors and doesn't osscilate, but might if one subbed the wrong components or had a bad board layout problem.
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The film version came into being because people said that film caps were better because 'everybody knows they are better'.
However, the smart thing to do is always to select components for the application you want to use them for.

Film caps in decoupling circuits together with other reactive circuit elements like wiring L lead to local oscillations.
So, using film caps here is ill advised; you can only decouple and damp if you remove energy and that's what an electrolytic does very well. You NEED the absorbtion factor to decouple and damp any ringing.
Film caps do this much less so you end up using the wrong cap and compensating for it with yet another network of a C and an R in the form of a snubber.

jan didden
 
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